bump version to 2.0.0

- Critical change that ensures reliablity and **should** have no bad side
  effects, but when you see any, please report:
  DICT, LIST, SET and Data List nodes (plus the DICT/LIST-taking STRING nodes)
  now degrade gracefully when their data source is not connected (`None`) instead
  of raising: read/query nodes return their natural "no data" result (the `get`
  default, an empty list, `False`, `0`, `None`), and transform nodes behave as if
  given an empty container.
- Add a "continue if not empty" flow-control node: it passes a value through
  unchanged when it is non-empty, and blocks execution (via an execution blocker)
  when the value is empty. It is list-aware (`INPUT_IS_LIST` / `OUTPUT_IS_LIST`),
  so it is immune to ComfyUI's empty-input batching crash and passes data lists
  through unchanged. By default the block is silent; an optional `message` can be
  set to show a dialog. Use it before nodes that cannot handle an empty input
  (e.g. a node receiving an empty Data List) to stop execution instead of failing.

src/basic_data_handling/string_nodes.py

@@ -371,6 +371,7 @@ class StringFormatMap(ComfyNodeABC):
     FUNCTION = "format_map"
 
     def format_map(self, template, mapping):
+        mapping = mapping if mapping is not None else {}
         try:
             result = template.format_map(mapping)
             return (result,)
@@ -778,6 +779,7 @@ class StringDataListJoin(ComfyNodeABC):
 
     def join(self, sep, strings):
         separator = sep[0]    # everything comes as a list, so sep is list[str]
+        strings = strings if strings is not None else []
         return (separator.join(strings),)
 
 
@@ -806,6 +808,7 @@ class StringListJoin(ComfyNodeABC):
     FUNCTION = "join"
 
     def join(self, sep, strings):
+        strings = strings if strings is not None else []
         return (sep.join(strings),)